Studies will be conducted on suckling rabbits to define the role of lactose feedings on developmental changes in intestinal permeability to alimentary proteins. Suckling animals have negligible gut epimerase activity. It is hypothesized that lactose is required for adequate glycosylation of epithelial surface glycoproteins and glycolipids. Through this role, the sugar would be a major determinant of epithelial developmental changes in receptor-mediated functions, such as response to trophic factors and endocvtosis, and would also be an important contributor to membrane stability. Suckling rabbits will be artificially-fed with formulas containing lactose, glucose and fructose, or glucose alone. Jejunal epithelium and isolated epithelial cells will be obtained from animals of ages 1, 3, 6, 12 and 19 days, as well as adults. The interactions of EGF, insulin, Concanavalin A, RCA120, and soybean lectin with the opithelial cell will be examined with respect to developmental progress of the following parameters: 1) effect on transport of Yield-amino-isobutyrate, 2) binding characteristics of the brush border membrane, 3) lectin-induced fluid-phase endocytosis of glycinin (a soy globulin), and 4) membrane resistance to lysis by soyasaponins, as well as other amphiphiles, and lytic-induced internalization of glycinin. Finally, jejunal epimerase activity and carbohydrate composition of the epithelial brush border will also be determined. Animals fed lactose-free formulas are expected to exhibit high epithelial reactivity to ConA and poor or absent responses to RCA120 and soybean lectin. Breast-fed and lactose-fed animals should show the opposite pattern. Comparison of the effects of these three lectins on epithelial internalization of soy protein in lactose and non-lactose fed animals should give insight into the relationships between intestinal glycosylation and membrane permeability during early development. These studies are expected to contribute to the understanding of alimentary protein intolerance in infants.